SURFACE TREATMENT OF SONOTRODE MATERIAL AW 7075

Transcription

1 SURFACE TREATMENT OF SONOTRODE MATERIAL AW 7075 EMMER Štefan 1, BAKSA Peter 2, KOVÁČIK Jaroslav 34 Keywords: Electro spark deposition, substrate AW 7075, metal-ceramic functional layers, micro hardness, micro roughness, nano roughness Abstract Electro spark deposited TiB 2 functional layer and composite functional layers Ni-TiB 2 and INC713LC-TiB 2 on the aluminium alloy substrate AW 7075 were evaluated with respect to achieved microhardness, roughness, and microstructure. The thicknesses of the layers were observed in the range of 30 to 50 microns. Hardness of the layers (HV) is several times higher than the hardness of the aluminium alloy substrate AW INTRODUCTION Currently, Ti6Al4V titanium alloy and aluminium alloy AW7075 may be considered as the best sonotrode materials. The basic problem solved through sonotrode materials is their hardness and wear resistance, i.e. properties required for their use as sonotrode tool materials. A possible solution for increasing their required properties is through their surface finishing. At the beginning of this century Electro Spark Deposition (ESD) [1-4] attracted increasing interest thanks to the possibility of its use for the production of hard and wear resistant surfaces on different kinds of materials in engineering. Essentially the ESD process can be defined as a kind of pulsed micro welding, since the material of the working electrode is deposited on the metal substrate by the short circuiting of the electrode with the substrate, when an electric arc of short time duration at high currents and low voltages is created. The biggest advantage of the ESD method is the possibility to melt and deposit a material with a high melting point, so that a metallurgical bond between the deposited material and the substrate can be achieved all with only a small heat affected zone in the substrate. The paper deals with an evaluation of the basic characteristics of the functional layers of TiB 2, Ni-TiB 2 and INC713LC-TiB 2 which were prepared by ESD deposition on aluminium alloy AW7075. Evaluated properties are microhardness, roughness, nano roughness, specific surface area and structure of the functional layers. 4 1 Institute of Technologies and Materials, Faculty of Mechanical Engineering, IVMA STU, Slovak University of Technology, Pionierska 15, Bratislava, Slovak Republic, stefan.emmer@stuba.sk 2 Slovak University of Technology in Bratislava, Faculty of Mechanical Engineering, Institute of Technologies and Materials, Pionierska 15, Bratislava, peter.baksa@stuba.sk 3 Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, Račianska 75, Bratislava, Slovak Republic, ummsjk@savba.sk

2 2 EXPERIMENTAL The experimental samples were prepared from the substrate material AW7075, onto which a layer of TiB 2 or composite layers of Ni-TiB 2, INC713LC-TiB 2 were deposited by ESD technology. In the case of the composite layer, deposition was carried out as follows: at first the Ni layer was deposited, then the layer of TiB 2, or the layer of INC713LC and then thetib 2 layer. ESD parameters which were monitored during the deposition of layers were voltage, power of equipment, sparks frequency, speed and direction of electrode rotation (electrode made from TiB 2, INC713LC and Ni). The deposition of the layers was performed in a protective atmosphere (argon). AW 7075 is one of the strongest aluminium alloys which can be heat treated after manufacturing. TiB2 is an extremely hard ceramic compound that has excellent resistance to mechanical wear (Table 1.). Table 1: Physical and mechanical properties of TiB2 at 20 C. Density [g.cm -3 ] 4.52 Young modulus [GPa] Melting point [ C] 2970 Poisson ratio Bending strength [MPa] Resistivity [Ω.cm] Hardness HV 3400 Thermal conductivity [W/m.K] 25 Technical grade Ni (Nickel 200 (99.5Ni)) in a compact state is air-stable at room temperature, has high corrosion resistance and stability in air at elevated temperatures. Creep resistant alloy INC713LC is characterized by a strong resistance to creep and thermal fatigue. Microstructures of the layers were investigated using an EDX analyser which belongs to the scanning electron microscope Jeol JSM X-ray analyses were performed on the device D8 ADVANCE. Roughness was evaluated using a confocal microscope Zeiss LSM 700, and nano roughness was determined using atomic force microscopy on the device NT-MDT SOLVER P47 PRO. 3 MICROHARDNESS Microhardness was measured on an AW 7075 substrate and also on deposited layers of TiB 2, Ni-TiB 2 and INC713LC-TiB 2. Determination of microhardnes was done using an INDENTAMET 1100 SERIES device and the measurement results were processed using Omnimet software. Inn Fig. 1 is a graphical representation of the measured average values of microhardness for the substrate and composite layers prepared by electro spark deposition (see Fig. 2).

3 Figure 1: Microhardness of substrate AW 7075 and deposited layers of TiB 2, Ni-TiB 2 and INC713LC-TiB 2. Figure 2: Left - surface layer of Ni-TiB 2 on the substrate AW7075, right - surface layer of INC713LC-TiB 2 on the substrate AW7075 (SEM) 4 MEASUREMENT OF MICRO ROUGHNESS AND NANO ROUGHNESS OF SURFACE LAYERS Measurement of the surface micro roughness of deposited layers of Ni-TiB 2 and INC713LC-TiB 2 was performed using a laser scanning confocal microscope Zeiss LSM 700 with laser wavelength = 405 nm and 20x zoom of the used lens. The measured area of the samples was 1230 x 1230 m for 2D topography and 320 x 320 m for 3D topography (see Fig. 3 and Table 2.).

4 Figure 3: View of 3D micro topography of the scanned surface layer INC713LC-TiB 2 (X-axis) before the transformation to fit plane and the corresponding profile curve. Table 2: Micro roughness of Ni-TiB 2 and INC713LC-TiB 2 layers. layer Ni-TiB2 on AW 7075 INC713LC-TiB2 on AW 7075 Average surface roughness W Sa [μm] Table 3: Nano roughness of Ni-TiB 2 and INC713LC-TiB 2 layers (measured area 2x2 m). layer Ni-TiB2 on AW 7075 INC713LC-TiB2 on AW 7075 Average surface roughness S a [nm] Nano roughness was measured on an NT-MDT Solver P47 PRO atomic force microscope. Measurements were made in semi-contact mode using silicon needles NSG03 - R 5-10 nm. On the samples was measured surface area 1x1 m, 2x2 m (Fig. 4 and Table 3.) and, for the sample with a Ni-TiB 2 layer, also the surface area 0.5 x 0.5 m. Figure 4: 3D nano topography of the scanned surface of layers INC713LC-TiB 2 (left) and Ni-TiB 2 (right) measured surface area 2x2 m.

5 Figure 5: EDX semi-quantitative line analysis of the elements: INC713LC-TiB 2 layer on AW7075 substrate and the corresponding line profile. Figure 6: X-ray analysis of INC713LC-TiB 2 layer on AW7075 substrate with observed peaks of Al 3 Ti, Al and TiB 2 phases.

6 5. DISCUSSION OF THE RESULTS OBTAINED AND CONCLUSION Electro spark deposited coating TiB 2 and composite layers Ni-TiB 2 and INC713LC-TiB 2 enabled to increase the surface hardness of aluminium alloy AW 7075 from hardness of 178 HV01 up to 1163 HV01 for layers INC713LC-TiB 2, which represents an increase of hardness by a factor of 6.5. Roughness was evaluated on micro and nano scales. The measurement results showed that in the future it will be sufficient to perform the micro roughness measurements, as they clearly reflected the different levels of micro roughness for the investigated ESD layers of Ni-TiB 2 and INC713LC- TiB 2 on the AW7075 substrate. Although similar results were obtained for nano roughness measurements, since the investigated surfaces are of smaller order of magnitude, this makes it hard, in some cases, to precisely determine the actual roughness. It was shown that due to the use of ESD layers in the surface treatment, especially in industrial practice, nano roughness will be used only in specific cases. It must be noted that in the case of ESD layers surface roughness depends on several factors, such as the parameters of deposition, but also significantly on the physical and chemical properties of the substrate. Structural and chemical microanalysis confirmed the presence of a hard TiB 2 phase in all prepared layers (Fig. 5). In addition to the hard TiB 2 phase in all investigated layers, other reaction phases were found, originating from the deposition process such as Al 3 Ti and Al (see Fig.6 for example). This implies that after deposition the structure of the ESD layer exhibits a certain degree of structural and chemical heterogeneity. This heterogeneity is dependent on the number of physical and metallurgical properties of the substrate material, the material of the working electrode, the ESD environment and, ultimately, on the cooling rate of the micro volume of the deposited melts. ACKNOWLEDGEMENT The authors thank for the financial support the Slovak grant agency APVV for the project VMSP II , and the VEGA Agency for projects 1/0383/15 and 2/0158/13. REFERENCES [1] R.N. Johnson and G.L. Sheldon. Advances in the electro spark deposition coating process. J. Vac. Sci. Technol. A 4 (6), (1986) [2] J.L. Reynolds, R. L. Holdren and L. E. Brown, Electro-Spark Deposition, Advanced Materials and Process 161(3), ASM, International, Materials Park, Ohio. (2003) [3] K.R. Chan, N. Scotchmer, J. Zhao, and Y. Zhou. Weldability Improvement Using Coated Electrodes for RSW of HDG Steel. SAE Technical Paper (2006) [4] R.J. Wang, Y.Y. Qian, and J. Liu, Structural and interfacial analysis of WC92-Co8 coating deposited on titanium alloy by electro spark deposition, Applied Surface Science 228 (2004)